This invention relates generally to methods and apparatus for reconstruction of computed tomography (CT) images, and more particularly to methods and apparatus for rapid acquisition of projection data for high resolution reconstruction of CT images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the object being scanned is not moved, and the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Helical scanning is used in some CT imaging systems, as are multislice detectors. During helical scans, a patient is placed on a moving table, which transports the patient through an opening in the rotating gantry. The direction of movement is referred to as the z-axis of the imaging system, and a multislice detector of such a system has multiple parallel rows of detector elements. The rows themselves are stacked in the z-direction, so that projection data acquired by each row corresponds to a xe2x80x9cslicexe2x80x9d of a patient. For axial scans, the plane of each slice is perpendicular to the z-axis if an axial scan is performed. The detector elements of adjacent rows of known multislice detector arrays abut one another, and are characterized by a spacing distance between their centers. When a helical scan is performed, the table moves a selectable distance during one gantry rotation. The ratio of the z-axis movement during one gantry rotation to the z-axis spacing between centers of the detector elements in each row is defined as the xe2x80x9chelical pitchxe2x80x9d that characterizes a given scan. (For a single row detector array, the z-axis spacing is replaced by the thickness of the single row in the z-axis direction.)
During a helical scan, projection data is collected during transport of the patient through the rotating gantry. With appropriate image reconstruction techniques, helical scans provide a relatively efficient way of imaging a volume of a patient that is thicker along the z-axis than the combined thickness of the rows of a multislice detector.
In some CT imaging applications such as cardiac imaging, a body part being imaged is not stationary. In the cardiac imaging case, it is necessary to use EKG gating to reconstruct images with data from a particular phase of a cardiac cycle. However, imaging an entire heart typically requires scanning a patient volume having a thickness of 12 cm, which is quite large in relation to the total thickness that can be imaged by known multislice CT detector arrays. In addition, projection data from a sufficient span of view angles is required for CT image reconstruction of any selected cardiac cycle phase. These requirements work to reduce the maximum helical pitch that can be used for cardiac scanning. However, with a low pitch helical scan, it may be difficult for a patient to hold his or her breath long enough during the scan to avoid additional body movement that would reduce the resolution of reconstructed images. Also, because low pitches translate directly into longer scanning times, patient dose is increased.
One known method for efficiently reconstructing image data from helical scans is known as a xe2x80x9chalf scanxe2x80x9d reconstruction method. This method takes advantage of redundancy inherent in scanned data by using only projection data acquired during one-half rotation (180 degrees) of the CT gantry plus one fan angle. (A fan angle is defined as the maximum angular extent of the acquired projection data, which depends on the angular width of the CT radiation beam and/or the angular extend of the detector array.) However, known reconstruction methods utilizing helical scan half scan reconstruction use the data from all the rows of detector elements to produce a single image per cardiac cycle rather than multiple images per cardiac cycle.
For example, and referring to the representation of FIG. 3, at least one known CT imaging system with a four row detector produces only one image (i.e., one slice) per cardiac cycle. In FIG. 3, the vertical axis units are gantry rotations, while the horizontal axis unit is the distance between centers of detector elements in adjacent rows (in this case, 2.5 mm). Thus, time is represented on the vertical axis and z-axis distance is represented on the horizontal axis. Solid diagonal lines 102, 104, 106, and 108 represent z-axis positions of a patient scanned by each row of detector elements as a function of time, for a 3:1 pitch. In FIG. 3, the gantry speed is 0.8 seconds per rotation for a heart rate of 75 bpm, or 1.0 seconds per rotation for heart rate of 60 bpm. Projection data acquired during the time indicated by vertical bars 110, 112, 114, and 116 is used to reconstruct an image corresponding to a selected phase of the cardiac cycle of the patient being scanned. Projection data acquired four detector rows is interpolated to points on vertical bars 110, 112, 114, and 116, the centers of which lie on a midpoint of the multislice detector, which is represented by diagonal dashed line 118.
When a complete 12 cm of coverage is desired in a single breathhold of 30 seconds, reconstruction of a diastole phase or any other phase results in gaps 120 between two adjacent images of the same phase. At the 3:1 pitch represented in FIG. 3, a space of 7.5 mm is created between images. Alternatively, the pitch can be reduced, but then more than one patient breathhold will be necessary to obtain 12 cm of coverage, and patient dose is significantly increased.
It would therefore be desirable to provide methods and apparatus for reducing patient dose and for reducing gap distances between images in cardiac CT imaging scans.
There is therefore provided, in one embodiment of the present invention, a method for reconstructing cardiac images using a computed tomographic (CT) imaging system. The method includes steps of: selecting a helical scanning pitch for scanning a patient; scanning the patient, including the patient""s heart, with a computed tomographic imaging system having a plurality of detector rows and a rotating gantry to acquire projection data from the plurality of detector rows; selecting a phase of the cardiac cycle for imaging; combining portions of the acquired projection data from a plurality of detector rows, the combined portions corresponding to the selected cardiac phase; and reconstructing images, including images of the patient""s heart, from the combined, interpolated projection data.
This and other embodiments of the present invention are effective in reducing patient dose by allowing helical scans at higher pitches, and for reducing gap distances between images in cardiac CT imaging scans.